The invention relates generally to telephone transmission systems and in particular to novel apparatus for feeding a telephone subscriber loop.
Telephone systems typically employ balanced twisted wire pairs for audio transmission to and from subscriber telephone sets. Such twisted pairs are referred to as subscriber line loops. The characteristics of the twisted wires is such that telephone circuits are usually designed to match a 600.OMEGA. AC impedance for the audio signals to minimize standing waves which cause undesirable echoes. In addition to audio, DC power is carried on the same twisted pair to power the telephone set and to provide various signalling information (busy, idle, etc.). Nominal impedances other than 600.OMEGA. are encountered in subscriber loops served by telephone central offices, and in practice the actual loop impedance may vary from the nominal impedance.
Thus, the feed point end of the subscriber loop, i.e., the end remote from the subscriber telephone set, requires a circuit that couples audio in and out of the two wire pair as well as DC power. The audio and DC must not interfere with each other and an AC impedance match to the loop is required.
Most prior art loop feed arrangements include a hybrid transformer for coupling the audio signals to and from the subscriber loop. In one widely used prior art arrangement, known as a "new hybrid", the DC loop power is applied through the hybrid transformer windings such as in FIG. 1. Such an arrangement requires a large transformer core due to the high saturation resulting from the large DC current carried.
A variation of the "wet hybrid" arrangement is a loop feed in which the size of the hybrid is substantially reduced by blocking the DC current from reaching the hybrid, thus requiring the hybrid to handle only relatively low level audio signals. In this arrangement, shown generally in FIG. 2, the DC loop power is applied through two inductors which present a relatively low DC resistance, so as not to increase substantially the DC power requirements. The AC impedance presented to the subscriber loop is determined primarily by the terminating impedances seen by the transmit and receive ports of the hybrid and the hybrid balance network. The inductors' AC impedance is sufficiently higher than the loop impedance so that they have negligible effect. A disadvantage of this approach is that the physical size of the inductors is large making it difficult to provide a multiplicity of loop feed circuits in a small physical package. Also such inductors are expensive, adding to the overall cost of the system.
Another type of prior art line feed circuit employs a hybrid transformer and a pair of DC current regulators to apply DC loop power. This approach is relatively expensive and the current regulators are difficult to balance.
In a further type of prior art line feed circuit, an electronic hybrid employing operational amplifiers is provided. High valued resistors are used to feed DC power to the line. However, such resistors require a higher than standard voltage DC source to provide enough current to drive longer loops.